1. Field of the Invention
The present invention relates to a Raman amplifier that transmits and Raman-amplifies signal light, an optical transmission system in the relay section of which the Raman amplifier is provided, and an optical fiber that is used for Raman amplification in the Raman amplifier.
2. Related Background Arts
An optical fiber amplifier amplifies signal light in the optical fiber and compensates for the loss when the signal light propagates through an optical transmission line, and is equipped with an optical fiber for amplification of light and means for introducing pump light.
Pump light is introduced through the pump light introducing means into the optical fiber for the light amplification, and the signal light propagating through the optical fiber for the light amplification is amplified and output.
There are two kinds of optical fiber amplifiers: a rare-earth-doped optical fiber amplifier which uses the stimulated emission phenomena of rare earth elements such as Nd and Er, and a Raman amplifier which uses stimulated Raman scattering phenomena.
The rare-earth-doped optical fiber amplifier is modularized and provided in a repeater. On the other hand, the Raman amplifier amplifies signal light with the optical transmission line or the part thereof itself acting as an optical fiber for the light amplification (optical fiber for Raman amplification). In the optical transmission line that uses the Raman amplifier, it is possible not only to reduce the effective loss of the optical transmission line (=transmission lossxe2x88x92Raman amplification gain) but also to restrain the occurrence of the nonlinear optical phenomena which is caused when the power of the signal light becomes too great.
The Raman amplifiers are described in Literature 1: T. N. Nilesen, et al., xe2x80x9c1.6 Tb/s (40xc3x9740 Gb/s) Transmission Over 4xc3x97100 km Nonzero-Dispersion Fiber Using Hybrid Raman/Erbium-Doped Inline Amplifiersxe2x80x9d, ECOC ""99, PD2-2 (1999) and European Patent Application Publication No. EP 0911926 A1, etc.
The Raman amplifier described in Literature 1 is an amplifier which Raman-amplifies signal light of 1.55 xcexcm wavelength band propagating through a dispersion-shifted optical fiber that is an optical transmission line, by introducing pump light of 1.45 xcexcm wavelength into the dispersion shift optical fiber. The invention described in EP 0911926A1 is directed to an optical amplifier for wide wavelength band in which a Raman amplifier and an Erbium-doped fiber amplifier (EDFA) are combined. The Raman amplifier used therein Raman-amplifies signal light of the 1.55 xcexcm wavelength band propagating through the optical fiber for the transmission line and compensates gain deviation of EDFA by introducing pump light of 1.51 xcexcm or 1.53 xcexcm wavelength into the optical fiber for the transmission line.
In a Raman amplifier it is important to make an optical fiber for Raman amplification as long as possible and to make a Raman amplification gain as great as possible. Therefore, it is necessary to enlarge the power of the pump light to be introduced into the optical fiber for Raman amplification or to minimize the transmission loss of the optical fiber for Raman amplification at the pump light wavelength.
As for the Raman amplifiers described in Literature 1 and EP 0911926A1, however, the loss of the pump light is great because the signal light wavelength is at 1.55 xcexcm wavelength band in which the transmission loss of the optical fiber for Raman amplification becomes the smallest and the pump light wavelength is in the range of 1.45 xcexcm to 1.53 xcexcm. Furthermore, there is a limit to increasing the power of the pump light which is to be introduced into the optical fiber for Raman amplification because the power of light that is output from a semiconductor laser light source which is generally used as a pump light source is limited and because a light amplifier that can amplify light in the wavelength range of 1.45 xcexcm to 1.53 xcexcm is not yet available. Therefore, there is also a limit to increasing a Raman amplification gain in a Raman amplifier.
A Raman amplifier according to the present invention is provided with an optical fiber for transmitting signal light and Raman-amplifying the signal light by means of pump light introduced thereinto. It is also provided with a means for introducing into the optical fiber for Raman amplification light, as pump light, whose wavelength is within the amplification wavelength band of EDFA. Thus, the wavelength of the pump light for Raman amplification becomes the wavelength that minimizes the transmission loss in a silica-based optical fiber. Therefore, the pump light for Raman amplification propagates through the optical fiber for Raman amplification at a low loss, and the Raman amplification gain can be increased in the Raman amplifier accordingly.
An optical transmission system according to the present invention is characterized in that (1) a Raman amplifier is provided in a repeater section thereof, (2) the Raman amplifier is equipped with an optical fiber for transmitting signal light and Raman-amplifying the signal light by means of pump light introduced thereinto, as well as with a means of introducing light, as the pump light, whose wavelength is within the amplification wavelength band of EDFA into the optical fiber for Raman amplification and (3) the optical fiber for Raman amplification constitutes a part or the whole of the optical transmission line of the repeater section.
In this optical transmission system it is possible to attain an excellent transmission characteristic by making the power of signal light which reaches the receiving end to be sufficiently large while restraining nonlinear optical phenomena from occurring because of the signal light power becoming too great in each place of the optical transmission line. Moreover, the Raman amplification gain of the signal light in the Raman amplifier can be made sufficiently great to perform a long distance optical transmission.
The optical fiber according to the present invention typically has an absolute value of chromatic dispersion in the range of about 0.1 to 10 ps/nm/km at 1.65 xcexcm wavelength. By using this optical fiber as an above-mentioned optical fiber for Raman amplification, an excellent transmission characteristic can be attained because the waveform distortion of the signal light can be restrained from occurring due to the accumulation of chromatic dispersions in the optical fiber for Raman amplification and because the interaction between channels by the nonlinear optical effect in wavelength-multiplexing transmission can be restrained.
The optical fiber according to the present invention is characterized in that its effective area at 1.55 xcexcm wavelength is equal to or less than 85% of its effective area at 1.65 xcexcm wavelength. By using the optical fiber of the present invention as above-mentioned optical fiber for Raman amplification, the Raman amplification gain becomes great because the nonlinearity of the optical fiber for Raman amplification is great at a pump light wavelength, and deterioration of the transmission characteristics of the signal light due to the nonlinear optical effect can be restrained because the nonlinearity of the optical fiber for Raman amplification is small at a signal light wavelength.
The above and further objects and novel features of the invention will be more fully clarified from the following detailed description when the same is read in connection with the accompanying drawings. It is to be expressly noted, however, that the drawings are for the purpose of illustration only and not intended for a definition limiting the scope of the invention.